One more thing, Like I said in post 3. A stuck valve or broken spring can do this too. I would take the valve body plates out, inspect the for broken springs, and stuck valves. Use the diagram below:

http://www.sonnax.com/downloads/valve-body-layouts/VBL-Mercedes-722-6.pdf

This is a no brainer to do, and worth the effort. I believe the valve body does not have to be removed to this.

Martin